Machine for producing printing plates with variable reproduction scale



May 30, 1961 HELL MACHINE FOR PRODUCING PRINTING PLATES WITH VARIABLEREPRODUCTION SCALE Filed May 8, 1958 United States Patent MACHINE FORPRODUCING PRINTHWG PLATES WITH VARIABLE REPRODUCTION SCALE Rudolf Hell,Kiel, Germany, assignor to Dr.-Ing. Rudolf Hell Kommanditgesellschaft,Kiel-Dietrichsdorf, Germany, a German company Filed May 8, 1958, Ser.No. 733,971

Claims priority, application Germany June 4, 1957 6 Claims. (Cl.178-6.6)

This invention is concerned with a machine of the twotable type forelectromechanically producing printing plates with continuously variablereproduction scale.

Systems are known for electromechanically producing printing plates,whereby the reproduction scale, that is, the ratio between dimensions onthe printing plates and on the copies to be reproduced, can beselectively and adjustably changed. Thus, there are methods and devicesknown in which the copy and the printing plate blank are respectivelymounted each upon a rotating drum, whereby both drums are advancedaxially, with the scanning and engraving elements in fixed positions, orelse wherein the two drums are in fixed position, while the scanning andengraving elements are advanced with respect to the circumference of oneof the drums. The scanning and engraving is efiected either alongequidistant circles or along helical lines. The enlargement or reductionof the scale on the printing plate, as compared with the scale of thecopy to be reproduced, is obtained either by the use of drums ofdifierent diameter, to be selectively used for one of the drums, andoperating both drums at identical speed of rotation, or by using drumsof identical diameter but rotating them at diflerent speeds. The ratioof the advance steps per drum revolution and advance speeds of thescanning and engraving elements, corresponding respectively to theenlargement and reduction and the selected screen, is produced bycorresponding step-up and step-down gear means.

There are also systems known, comprising horizontally extending tablesadapted to reciprocate in parallel planes, one table carrying the copyto be reproduced, which is scanned by a scanning element point for pointin successive lines, the other table carrying the printing plate blankto be processed which is simultaneously engraved, by a suitable tool,point for point along successive lines. Upon conclusion of the scanningand engraving of each line, the scanning and engraving elements eachexecute a stepwise line advance motion, perpendicular to the scanningand engraving directions, by the distance of two scanning lines andengraving lines, respectively, such spacing corresponding respectivelyto the screening applied and to the reproduction scale desired. Theenlargement or reduction of the scale on the printing plate, as comparedwith the copy to be reproduced, is thereby effected by step-up andstep-down gears or by lever mechanisms respectively interposed betweenthe drives of the two tables and the drives for the line shifting oradvance of the scanning and engraving elements.

It is further known to impart either to the scanning or to the engravingelement or to both auxiliary motions in the scanning and engravingdirections, respectively relative to drums of identical diameterrotating with identical speed of rotation or relative to tablesreciprocating with identical speed, and employing suitable step-up andstepdown gears for the purpose of producing the desired ratio of lineadvance or shifting motion of the scanning and engraving elements.

"ice

Another known system uses, instead of two separate tables respectivelyfor the copy to be reproduced and for the printing plate blank which isto be engraved according to variable reproduction scale, a singlereciprocating table carrying upon one side the copy to be reproduced andupon the other side thereof the printing plate blank which is to beengraved. In such known system, either the scanning element or theengraving element executes relative to the reciprocating table inauxiliary motion in the respective scanning and engraving direction orelse, both elements execute such auxiliary motions.

A further known system is of the hybrid table-drum type, in which aflexible printing plate foil is wrapped about a rotating drum, anengraving head being moved with respect thereto parallel to the drumaxis, such head carrying a heated tool for engraving the foil along ahelical line by burning out the screen elements. The copy to bereproduced is carried by a horizontally disposed table which executes atranslatory motion parallel to the drum axis. The copy to be reproducedis scanned by an optical scanning device swinging forth and back about afixed axis, in a plane perpendicular to such axis, along successivelines transverse to the drum axis. The reproduction scale is changed byalteration of the ratio between the swinging amplitude of the scanningsystem and the constant speed of rotation of the drum, and bycorresponding alteration of the ratio between the speed of translatorymotion of the table carrying the copy and the invariable translatoryspeed of the scanning head along the drum.

Other known systems project by optical means including a picturingobjective, an enlarged or reduced picture image of the copy to bereproduced onto a frosted plate or screen, and such projected picture isphotoelectrically scanned point by point along successive lines, therebyproducing fluctuating photoelectric currents for controlling the depthof penetration of an engraving tool which engraves a printing plate of asize corresponding to the optically projected picture, point by point insuccessive lines.

Further systems have become known in which the enlargement or reductionof scale on the printing plate as compared with the scale of the copy tobe reproduced, is likewise carried out by means of a picturingobjective, without, however, Projecting the copy to be reproduced in itsentirety, as in the system noted in the preceding paragraph, but insteadscanning the original copy by a dot of light whose diameter and pathcorrespond to the optically enlarged or reduced image of the diameterand path of a punctiform light source, such source being fixedlyconnected with the engraving system which engraves a printing plateblank simultaneously with the scanning of the copy, the depth ofpenetration of the engraving tool being controlled by means of aplurality of photocells fixedly disposed in the neighborhood of theobjective and ascertaining the relative brightness of the picture pointwhich is being scanned upon the copy.

Finally, a system has become known in which the alteration of thereproduction scale on the printing plate, as compared with the scale onthe copy to be reproduced is effected by optical picturing of the copywith electrical means. The alteration of the reproduction scale isthereby obtained by picturing the copy to be reproduced, by means of apicturing optics with variable picturing scale, always in the identicalsize upon the photo cathode of a picture chopping tube, scanning theoptical picture upon the photo cathode line for line by an electronbeam, controlling the depth of penetration of the engraving tool in theprinting plate blank by the picture currents produced by the tube,controlling the deflection of the electron beam in one coordinatedirection by the relative motion of the engraving table with respect tothe engraving system in the engraving direction while controlling thedeflection of the electron beam in the coordinate directionperpendicular to the first direction by the relative line advance orshift motion of the engraving system with respect to the engraving tableperpendicular to the engraving direction, such, that one deflection voltage for the electron beam is proportional to the table stroke while theother deflection voltage is proportional to the line advance or shiftpath of the engraving system with respect to the engraving table.

Due to recent advances which have made it possible to engrave metallicprinting plates electromechanically, demands for such engraving havearisen with increasing frequency. Systems employing rotating drumscannot satisfy the requirements for the electromechanical engraving ofmetal plates on account of the relative rigidity of such plates. Systemsemploying tables or drums in which the screen elements are burned outfrom printing plate blanks made of foils of synthetic material, by meansof heated engraving tools are likewise unsuitable for the processing ofmetallic printing plates. Systems employing tables, in which thevariable reproduction scale is produced mechanically by means of step-upand stepdown gears have the drawback of permitting setting of thereproduction scale only in fixed increments or stages but notcontinuously. One of the oldest two-table machines for making printingplates, employing for continuous variation of the reproduction scale alever mechanism resembling a parallelogram, has the serious drawbackthat there is no linear relation between the motions of the two tablesbut a complicated non-linear relation,

so that the image on the printing plate will not agree in smallestdetails with the copy to be reproduced, thus furnishing a printing platecarrying an engraved image which is distorted as compared with thepicture copy.

The invention proposes a two-table machine employing lever mechanismsfor the continuous setting of the reproduction scale and for thecoupling of the similarity motion of both tables.

The invention avoids the previously noted drawbacks by coupling the twotable motions which are in identical sense in the engraving directionand also the two oppositely directed line advance or shift motions ofthe scanning and engraving heads perpendicular to the table motions, bytwo continuously adjustable similar swing-lever mechanisms, in such amanner, that the ratios between the two table strokes and the two lineadvance or shift strokes as well as the ratios between the two tablespeeds and the two line advance or shift speeds are, independent of theselected screening and at a selected reproduction scale, constant andequal to the reproduction ratio.

The machine according to the invention comprises a reciprocable copytable carrying the picture copy to be reproduced, a similarlyreciprocable engraving table swing levers disposed for motion, forth andback, in iden- Y tical direction within a vertical plane, means forrotatably journalling the lower ends of said swing levers at structuralparts of the machine, means for rotatably journalling the upper ends ofsaid swing levers at said engraving table and the copy table,respectively, a slide for interconnecting said swing levers, said slideextending in a plane perpendicular to the planes of said tables andparallel to the planes of said swing levers, means for positivelyguiding said slide for forth and back motion corresponding to thereciprocating motion of said tables,

selectively adjusting the respective pivot means longitudinally of thecorresponding swing levers in accordance with the desired reproductionscale, a double-armed horizontally extending lever for interconnectingsaid engraving head and said scanning head for the purpose of couplingsaid heads for the line advance motion thereof, means for rotatablyjournalling one end of said doublearmed lever at the engraving head andthe other end thereof at the scanning head at respectively verticallyextending axes, vertically disposed pivot means extending from astructural part of the machine for rotatably journalling saiddouble-armed lever intermediate the opposite ends thereof, and means forselectively adjusting the position of said last named pivot means withrespect to the opposite ends of said double-armed lever in ac cordancewith the desired reproduction scale.

The invention will now be explained with reference to the accompanyingdrawing, wherein- Fig. 1 shows in schematic manner essential parts of amachine according to the invention; and

Fig. 2 is a diagram to aid in explaining the kinematic operation of theswing-lever mechanisms.

In Fig. 1, numeral 1 indicates a rectangular horizon tally disposedengraving table guided in rails 2 and 3 for reciprocating motion,numeral 4 indicating the printing plate blank mounted on the table 1which is to be engraved in accordance with parallel or diagonalengraving cutting. Numeral 5 indicates a cylinder mounted on structuralparts of the machine, containing a piston 6 connected with the pistonrod 7 which is fastened to cars 8 extending from the engraving table 1.The cylinder 5 is provided with two orifices 9 and 10 formed therein towhich are connected hydraulic pressure lines 11 and 12 terminating in ahydraulic control device 13 for governing the feed and return ofhydraulic fluid so as to eifect reciprocation of the piston 6 andtherewith reciprocation of the engraving table 1. The drive of theengraving table 1 may also be effected by means of a rotating spindlethreadedly guided within a nut carried by the engraving table, withsuitable reversing gear means for reversing the direction of rotation atthe conclusion of engraving of each line. The reversal of the tablemotion is effected by suitable control means, always when the engravingtool reaches an edge of the printing plate blank.

Numeral 14 in Fig. 1 indicates the rectangularly shaped horizontallydisposed copy table which is guided in rails 15 and 16 for reciprocatingmotion in the same direction as the engraving table 1. The two tablesmay be positioned in the same plane or in difierent parallel planes orlevels. The latter arrangement of the tables in dilierent planes orlevels is advantageous for reasons of saving space; the two tables mayoverlap incident to their mot-ions, resulting in a shortening of themachine dimensions. The copy table 14 carries the copy or picture 17which is to be reproduced.

There are provided two swing-levers 18 and 19 which are respectivelyrotatably journalled upon axes 20 and pendicular to the, plane of thetables. At the upper ends of the levers are formed longitudinal slotsrespectively indicated at 22 and 23', stub shafts or pins indicated at24 and 25, extending respectively from the engraving table and the copytable and entering the respective slots. Incident to reciprocation ofthe tables, the swing levers 18 and 19 will be displaced with thecorresponding slots gliding along the respective stub shafts 24 and 25.The copy table 14 is in the illustrated example at a lower level thanthe engraving table 1, and the svw'ng-lever 19 is, accordingly, shorterthan the swing-lever 18, by the level diiference. The swing-lever 18 isalsopro vided with a longitudinal slot 26 and the swing-lever 19 isprovided with a similar longitudinal slot Z7.

The coupling of the swing-levers 18 and 19 and therewith the coupling ofthe. tables 1 and 14 is efiected by a slide 28 which is disposed in avertical plane extending in parallel to the plane of the swing levers.Bearings 29 and 30 provide positive guidance for the slide 28 in itshorizontal displacement, the slide, accordingly, executing areciprocating motion in parallel with the reciprocating motion of thetables 1 and 14. Vertically extending slots 31, 32 are formed in theslide 28 respectively in line with the two lever pivots 20, 21, whichare respectively engaged by pivots 33 and 34. Numerals 35, 36 indicatemeans for fixing the pivots 3'3, 34 in any adjusted position withrespect to the respective slots 31, 32. The positions of the pivots 33,34, are, accordingly, adjustable on the slide 28. The two pivots 33, 34extend respectively through the slots 26, 27 formed in the swing levers18, 19, these slots permitting sliding of the levers along the pivotsincident to angular displacement of the levers and resultingreciprocation of the slide 28, the swing-levers coincidently rotatingabout the pivots 3-3, 34. Scales 37, 38 are provided, respectivelycarried by the swing levers 18, 19, upon which may be set the distanceof the pivots 3-3, 34 from the axes 20, 21, in the vertical zeroposition of the two swing-levers, by means of the pointers 39, 40,respectively associated with the pivots 33, 34. The coupling of the twoswing levers 18, 19, by means of the slide 28, etfects a linearsimilarity motion of the two tables 1 and 14, such, that the ratios ofthe table strokes and speeds are at any instant constant and inaccordance with the adjusted reproduction ratio. It is possible, byalteration of the adjustment of the two pivots 33, 34 with respect tothe slots 31, 32 in the slide 28, to set the machine in accordance withreproduction ratio which is within certain limits steadily variable asdesired.

Considered kinematically, the slide 28 with its pivots 33, 34 which areadjustable slots 31, 32, constitutes a rod of variable length. Incidentto reciprocating motion of the two tables, this rod always remainsparallel to itself, and the vertical distances of its rotation andgliding pivots 33, 34, from the connecting line ofthe two lever axes20', 21, remain during the reciprocation constant. The horizontal pathsofthe two pivots 3-3, 34, considered from the vertical zero position ofthe two swing-levers, are always equal one to the other. However, theangles of the two swing-levers, measured from the vertical zero positionabout which they are displaced incident to the table motion, aregenerally speaking different. They will be equal to one another onlywhen the connecting line of the two pivots 33', 34 is parallel to theconnecting line between the axes 20, 21. However, the reproduction ratiois in this case not 1 to 1 since the tables 1 and 14 have differentconstant vertical distances from the lever axes 20, 21. Moreover, thereproduction ratio is not determined by the absolute amounts of thevertical spacings of the pivot 33 from the axis 20 and of the pivot 34from the lever axis 21, but by their ratio, thus making the levelsetting of one of the two pivots 33, 34, for obtaining a desiredreproduction ratio, arbitrary. However, in order to produce forfacilitating operation unambiguous conditions, the absolute amounts ofthe spacing of the pivots from the respectively associated lower leveraxis is in connection with the reproduction ratio 1 to l fixed by thescale marking l and 1 upon the scales 37 and 38.

In connection with all enlargements to appear on the printing plate ascompared with the picture copy, the pivot 34 is invariably set on theright hand scale to the scale mark 1 and the pivot 33 is set on the lefthand scale underneath the scale mark 1 the greatest enlargement beingefiected when the pivot 33 is set to the scale mark 4. In connectionwith all reductions to appear on the printing plate as compared with thepicture copy, the pivot 33 is set invariably on the mark 1 of the scale37, and the pivot 34 is set to a mark underneath 1, the greatestpossible reduction resulting when the pivot 34 is set opposite the scalemark 0.33. The

scales 37v and 38 are not provided with linear subdivisions.

The kinematic conditions incident to similarity of motion of the twotables are represented in Fig. 2 in abstract manner. The constantvertical distance of the guide axis 24 from the lower iever axis 20* isassumed to be c, and the constant vertical distance of the guide axis 25from.- the lower lever axis 211 is assumed to be d. It is furtherassumed that c d, that is, that the engraving table 1 moves in a levelextending above the level of the copy table 14. The adjustable spacingof the pivot 33 from the lever axis 20' shall be assumed to be a, andthe adjustable spacing of the pivot 34- from the lever axis 21 shall beassumed to be b. It shall further be assumed that a b, that is, that themachine is set for enlargement of the image on the printing plate ascompared with the picture copy, as contrasted with the conditionsillustrated in Fig. 1 in which it is assumed that reduction of the imageon the printing plate as compared with the picture copy, is to beefiected. The angle of swing of the lever 18 from its vertical positionshall be assumed to be on and the angle of swing of the lever 19' shallbe assumed to be 3. The slide 28 so far as its kinematic effect is.concerned may be considered to be substituted by a rod 28 whichinterconnects the pivots 33 and 34, thereby holding the spacingtherebetween, at an adjusted reproduction ratio, constant. Due to thepositive horizontai guiding of the pivots 33 and 34, upon angulardisplacement of the levers 18 and 19, the pivot 33 will be shifted by anamount alga, reaching the position 33', and the pivot 34 will be shiftedby an amount h1g5, moving into the position 34'. The shifting of thepivots 33, 34 are, due to the rigid interconnection therebetween, bymeans of the rod 28', equal to one another, that is and the rod 28 is inthe shifted position 28 in parallel with its initial position. Thestroke of the engraving table 1 is x=c.tgot

and the stroke of the copy table is y= fi Upon considering thisrelation, there will result for the ratios of the two table strokes xand y, that is, for the Since it was assumed that c d, therefore, k l.Accordingly, there will result an enlargement v 1 on the printing plateas compared with the picture copy, when k.b/a l; a reproduction scale 1to 1, that is, v=1, when k.b/a=l; and reduction of the printing plateimage as compared with picture copy v l, when k.b/a 1.

The foregoing considerations show that it does not depend upon theabsolute values of a and b but upon the ratios thereof. They also showthat there is a strictly linear similarity motion of the tables sincethe reproduction scale xzy is at fixediy set values a and b constant,that is, independent of x and y.

There are several possibilities for the longitudinal equalization of thetwo levers 18, 19 incident to their swinging motion. Instead ofelfecting the longitudinal equalization by means of the slots 22, 2.6and 23, 27, as shown in Fig. l, the swing-levers, to produce the sameeifect, may be journalled at 24, 25 rotatably but not shiftable, and theslots for the longitudinal equalization or compensation may be providedat the lower ends of the levers, so that the levers glide up and downwith respect to the axesv 20, 21. The second longitudinal compensationbetween the lower ends. must also be maintained in such embodiment bythe provision of the slots 26, 27. A third possibility for thecompensation in length resides in providing slots in the opposite endsof the levers 18, 19 for sliding cooperation with the shafts or axes 24,20 and 25, 21 incident to the swinging motion of the levers. However, insuch a case, the lever 18 and the lever 19 must be arranged merelyrotatable about the pivots 33 and 34, respectively, but not movablerelative to the intermediate lever slots 26, 27, contrary to thearrangement illustrated in Fig. 1, wherein the longitudinal compensationor equalization must be effected along the slots 26, 27 due to the fixedjournalling of the lower lever ends at 20, 21, so that the pivots 36, 34can slide relative to the intermediate slots 26, 27, and therewith alsorelative to the scales 37 and 38. This applies, of course, only in casethe two levers are fixedly journalled at their upper ends, thelongitudinal compensation being effected by slots formed in the lowerends thereof.

The guide shaft extending from the copy table 14 is connected with abushing 41 which may be clamped to the rail 43 in any adjusted position,by means of member 42. This coupling-uncoupling device makes it possibleto place the copy table 14 in the scanning direction in desired initialposition with respect to the en graving table 1 as well as with respectto the scanning head 50 which will be presently described. Thecouplinguncoupling device is very important, making it possible to placethe scanning head prior to engraving at desired points of the picturecopy so as to ascertain the brightest and the darkest points for theadjustment of the whiteblack level and, further, in order to make itpossible to scan a picture copy section lying at any desired area of thepicture copy. The coupling-uncoupling device may be also provided inconnection with the engraving table 1 to obtain similar efiects.

Numeral 44 indicates the engraving head containing a knownelectromagnetic drive system for actuating the engraving tool or stylus45, such stylus executing during the engraving up and down motionscontrolled in accordance with the brightness of the picture copy. Theengraving head 44 is fastened to the carrier 46 which is pivoted on thenut 48 at 47, whereby the engraving head may be lifted with respect tothe engraving table 1. The not 48 is either split midway thereof in theline shift or advance direction or is formed in the manner of a segmentso as to make it possible to separate the engraving head 44 from thethreaded spindle 49 for the purpose of placing it in the advancedirection in alignment with a desired point on the printing plate blank4. The threaded spindle 49- is intermittently rotated so as to move theengraving head upon conclusion of the engraving of each line in astepwise line shift or advance motion perpendicular to the engravingdirection. This intermittent rotation of the threaded spindle 49 may beeffected by means of a rotary ball wedge clutch device generally knownfrom free-Wheeling drives used, for example, in connection withbicycles. The advantage ofsuch a clutch drive is that it permitssteadily alterable adjustment of the angle of rotation of the lineadvance or shift spindle 49. The release of the stepwise advance motionis controlled by the reversal of the table motions,

that is, a control element, for example, a contact is in known manneractuated at the conclusion of the engraving of a line, efiFecting knownand suitable control means for reversing the table motion, and alsoeffecting actuation of the clutch device so as to advance the engravinghead 44 and the scanning head 50 in a direction perpendicular to theline engraving motion and line scanning motion, respectively, inposition for respectively scanning and engraving the next line.

In-case only a small number of fixed advance or shift steps is required,corresponding to a small number of fixed screen'elements, the abovedescribed clutch device for'the (invent the line shift or advancespindle may be were replaced by a plurality of known gears withdifi'erent numbers of teeth controlled by resilient stop pawls.

The scanning head 50, containing a known optical and photoelectricscanning device, serves for the line by line scanning of the picturecopy 17.

A horizontally extending two-armed swing-lever 51 is provided for thecoupling of the line advance or line shift motions of the engraving head44 and the scanning head 50. The lever 51 has at the end facing thescanning head 50' a longitudinal slot 52 formed therein, a verticallydisposed pin 53 extending from the scanning head 50 entering the slot52. Incident to line advance motion of the scanning head 50, the lever51 will be angular-1y displaced by the action of the pin 53 in the slot52. The opposite end of the lever 51, facing the engraving head 44, isjournalled for rotation about trunnion 54 which may be clamped, bymember 55, in position at any desired place along the dovetailed bar 56,such bar being disposed on the nut 48 in parallel to the line advancemotion controlled by the spindle 49. The corresponding coupling devicecomprising the members 54, 55, 56 permits placing of the scanning headin the line advance direction in any desired position with respect tothe engraving head. The adjustment made possible by this coupling deviceis of value in connection with making enlargements or reductions ofsections of a picture copy. The coupling device may be provided inconnection with the scanning head 50 instead of with engraving head 44to obtain similar efl ects.

The lever 51 has a longitudinal slot 57 formed therein intermediate itsopposite ends for accommodating a stub shaft 58 in sliding cooperationtherewith. The stub shaft 58 may be clamped to a stationary bar 60 whichis connected with a structural part of the machine and extends inparallel to the engraving direction, in any desired position withrespect to such bar, by clamping means 59, a pointer 62 extending fromthe stub shaft 58 cooperating with a scale 61 on the bar 60 to set andto indicate the desired reproduction ratio for the line advance motion,such ratio being generally equal to the reproduction ratio in theengraving and scanning direction. The lever 51 will be rotated about thestub shaft 58, incident to the line advance motion, the stub shaftsliding within the slot 57 and constituting the changeable or adjustablepivot for the lever 51. Enlargement of the image on the printing plateas compared with the picture copy will result when the pointer 62extending from the stub shaft 58 is placed at the right of the scalemark "1 since the left (changeable) arm of the lever will be longer thanthe right (changeable) arm thereof. Reduction of the image on theprinting plate will be eifected when the pointer 62 extending from thestub shaft 58 is placed to the left of the scale mark "1 because thelengths of the respective arms of the lever 51 will differ in oppositesense. The lengths of the two lever arms change steadily incident to theline advance motions, but the ratio remains at any instant constant andequal to the adjusted reproduction scale which is given by the ratio ofthe distance of the trunnion 54 from the stub shaft 58 and the distanceof the pin 53 from the stub shaft 58. The reproduction scale is set withthe lever 51 in zero position, that is, when the lever extends inparallel to the engraving direction.

The lever 51, instead of having its left end journalled at a fixed pointand having its right end slotted, as shown in Fig. 1, may have its rightend journalled at a fixed point and its left end slotted. The operativeeffect will be the same, namely, the stub shaft 58 which is fixedlyclamped to the rail 60, by means of member 59, will be slidable withinthe slot 57 in the lever 51. The lever 51 may also be disposed rotatableabout the stub shaft 58 but not shiftable with respect thereto, and thestub shaft and claw 63 may be slidable on the rail 60. The adjustment orsetting of the reproduction scale is efiected as de 9 scribed before,with the lever 51 in its zero position, that is, disposed parallel tothe engraving and scanning direction.

The lever 51 may also be provided with slots formed therein at both endsthereof and may be rotatable about the stub shaft 58 but notdisplaceable relative thereto, and the shaft 58 may be fixedly connectedwith the rail 60 by means of the claw 63. The reproduction scale may insuch case be set on the scale 61 in any desired position of the lever51.

The line shift motions of the engraving head 44 and of the scanning head50 are due to the two-armed lever 51 in opposite sense; the result is,that the printing plate blank will be engraved in mirror-picture imageof the picture copy, as it must be to obtain side correct reproduction.

Generally speaking, the same reproduction scale must be set on the scale61 as on the scales 37 and 38 to obtain a printing plate image whichcorresponds geometrically to that on the picture copy. However, it ispossible to set a reproduction ratio for the tables, on the scales 37and 38, different from that for the line advance or shift motions setfor the lever 51 on the scale 61. This is important in connection withmaking a printing plate from a picture copy bearing an image which isdilferently distorted in two directions perpendicular to one another andwhen such distortions are to be corrected. The reproduction scale, insuch a case, must be different in the two directions extendingperpendicular one to the other. Such distortion correction may, however,be considered an exceptional case. The requirement for producing aprinting plate bearing a distorted image of a picture copy may arise,for example, when it is desired to produce distortions for the sake ofcreating amusing or cartoon-like effects.

In the production of screen printing plates, the line advance or lineshift motion of the engraving head is regardless of the reproductionscale, solely dependent upon the selected number of screening lines. Thetotal number of scanning lines upon the picture copy is equal to thetotal number of engraving lines upon the printing plate. However, thedensity of the scanning lines, that is, the number of scanning lines perunit of length, will depend upon the selected number of screening linesas well as upon the reproduction scale.

Changes may be made within the scope and spirit of the appended claims.

I claim:

1. In a machine for producing printing plates having a reciprocable copytable carrying the picture copy to be reproduced, a similarlyreciprocable engraving table carrying a printing plate blank, a scanninghead for scanning the picture copy, an engraving head for engraving theprinting plate blank in accordance with the scanning of said scanninghead, means for advancing said scanning head and said engraving head ina direction perpendicular to the direction of reciprocating motion ofsaid copy table and said engraving table, the combination for efiectingvariable reproduction scale in the production of printing plates,comprising a pair of single-arm swing levers disposed for motion, forthand back, in identical direction within a vertical plane, means forrotatably journalling the lower ends of said swing levers at structuralparts of the machine, means for rotatably journalling the upper ends ofsaid swing levers at said engraving table and said copy table,respectively, a slide for inter-connecting said pair of swing levers,said slide extending in a plane perpendicular to the planes of saidtables and parallel to the planes of said swing levers, means forpositively guiding said slide for forth and back motion corresponding tothe reciprocating motion of said tables, pivot means for rotatablyjournalling said pair of swing levers on said slide intermediate theends of said levers, means for selectively adjusting the respectivepivot means longitudinally of the corresponding swing levers inaccordance with the desired reproduction scale, means associated withthe repective single-arm swing levers for effecting compensation of thelength of the latter, incident to displacement thereof, a double-armedhorizontally extending lever for interconnecting said engraving head andsaid scanning head for the purpose of coupling said heads for theadvance motion thereof, means for rotatably journalling one end of saiddouble-armed lever at the engraving head and the other end thereof atthe scanning head at respectively vertically extending axes, verticallydisposed pivot means extending from a structural part of the machine forrotatably journalling said double-armed lever intermediate the oppositeends thereof, means for selectively adjusting the position of said lastnamed pivot means with respect to the opposite ends of said double-armedlever in accordance with the desired reproduction scale, and meansassociated with said double-armed lever for effecting compensation ofthe length of the latter incident to displacement thereof.

2. A structure according to claim -1, comprising means cooperativelyassociated with said first and said second named pivot means forvisually indicating the reproduction scale respectively adjustedthereby.

3. A structure according to claim 1, wherein the means for compensationof length of said single-arm swing levers incident to displacementthereof is positioned at said pivot means and at the upper ends thereofwhich are respectively journalled at said engraving table and at saidcopy table.

4. A structure according to claim 1, wherein the means for compensationof length of said double-armed lever incident to displacement thereof ispositioned at said last named pivot means and at one of the ends of saidlever.

5. A structure according to claim 1, wherein said tables arereciprocated along mutually parallel and partially overlapping planes.

6. A structure according to cairn 1, comprising journal means forjournalling the upper end of one of said singlearm swing levers at saidcopy table, and means for selectively adjusting the position of saidjournal means with respect to such table.

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